Excessive mag drop on Bendix S4LN-21

Over the last few years, my left mag drop has been getting lower and lower during the runup of my Lycoming O-290-D2. The right mag is OK, but the left is now at the 175 RPM limit (Leaning the engine helps.) Both mags have 300 hrs. on them SMOH. I've put it off long enough so yesterday I pulled the mag and removed the cover over the points. The point gap was only ~0.010, much less than the 0.012-0.024 specification. The points weren't pitted or anything, but they had some whitish oxide on them. With my Fluke, I checked the capacitance of the condenser and it was 0.389 uF, which is greater than the 0.3uF limit. I also checked the leakage resistance and it was 5 nS (200 Megohms). That's gotta' be acceptable. Would the small point gap explain the failed mag check, or should I be looking for something else? It seems like in 300 hrs, the phenolic cam follower could have worn, which would account for the sub-normal point gap, right? Not having the e-gap setting tools, I took the mag (and a new set of points) over to the maintenance shop to have them replace the points and set the e-gap. They have the tools over there. I also took a bottle of Tri-Flow, in case the follower felt needs lubrication. The "official" lube is outrageously expensive ($60), and the Tri-Flow is rated up to 475F.

Does anybody have any experience with Bendix mags? I sure could use some guidance to keep me out of trouble. Thanks in advance.

point gap drives the e gap, which dictates the power produced by the mag and delivered to the coils primary winding (you want the primary circuit to open at the peak of the magnets current flow). Low input power creates a weak secondary winding output and therefore a weak spark that possibly is causing your problem.

I would reset the e gap and see if things improve. Note that setting the egap is not as simple as setting the point gap. You will need to make or buy a tool for setting the e gap. I made mine.

Larry

lr172 said:

You will need to make or buy a tool for setting the e gap. I made mine.

Click to expand...

That's awesome that you made your own tool. Did you make the rotor holding tool as well? I didn't want to spend $128 on buying the 11-8150-1 Magneto Timing Tool, and I couldn't find the 11-8465 Rotor Holding Tool anywhere. I just wimped out and took the mag to the shop across from my hangar. They've got the tools.

snopercod said:

That's awesome that you made your own tool. Did you make the rotor holding tool as well? I didn't want to spend $128 on buying the 11-8150-1 Magneto Timing Tool, and I couldn't find the 11-8465 Rotor Holding Tool anywhere. I just wimped out and took the mag to the shop across from my hangar. They've got the tools.

Click to expand...

The linked tool is not the one used to set the e gap on a slick. The timing tool is just a piece of .063 cut to a unique shape. No need for a rotor holding tool for setting the e gap. Not sure how much it would assist, but definately not necessary.

EDIT: Just realized that you have a Bendix and not a slick and I am sure the process is differrent.

Larry

No problem. I'm just glad somebody is reading my plea for help (wink). I found this:

As the magneto rotates, there is one definite position where the best efficiency is obtained. This is just as the edges of the armature move away from the pole pieces causing the magnetic field to collapse. It is at this position that the fibre heel on the points needs to have reached a position on the upward slope of the cam to start to open the points. This is known as the internal timing of the magneto and most manufacturers provided a means of fine tuning this position. It would have been adjusted to the optimum position as part of the manufacturing process and would not normally need to be changed unless parts such as the cam ring were replaced. Adjusting the points gap by moving the fixed half of the contact breaker points will cause the points to open when the fibre heel is higher or lower on the rising edge of the cam - closing the gap retards the timing, opening the gap advances it. In short, adjusting the size of the points gap alters the internal timing of the magneto.

Click to expand...

If I understand correctly, setting the internal timing of a mag involves setting the points to open when the magnets are 10 deg past "neutral". If the point gap was too low, I think that would make the internal timing more than 10 degrees, meaning the magnetic field would be more collapsed than desired, meaning a weaker spark. As part of my runup, I use my JPI graphic engine monitor in the "normalize" mode to observe the response of each cylinder when running on one mag. I have noticed that my cyl #3 doesn't rise quite as much on the Left mag. I'm not sure what that means, but will pull and inspect the plugs tomorrow. One would think that if the Left mag was weak, ALL the EGTs would be lower on that mag. Come to think of it, though, when performing my mag checks, I've only been looking for an even rise; I haven't checked whether the EGT rise is the same on each mag. Hmmmmm...